Techniques for providing broadcast services on spot beam satellites

ABSTRACT

Techniques for providing broadcast services on spot beam satellite are provided. These techniques enable the mission of a spot beam satellite system to be changed from providing spot beam transmission to broadcast transmissions, and vice versa, without requiring that the satellite be reconfigured. Broadcast data may be encoded and transmitted concurrently on a plurality of spot beams. According to some embodiments, the broadcast data may be encoded using a space-time code and/or forward error corrected (FEC) encoded to enable a receiver to correct errors in the signal received from the spot beam satellite.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a divisional application of U.S. Non-Provisionalpatent application Ser. No. 12/052,339, filed, Mar. 20, 2008, and titled“TECHNIQUES FOR PROVIDING BROADCAST SERVICES ON SPOT BEAM SATELLITES,”which claims benefit to U.S. Provisional No. 60/896,044, titled“TECHNIQUES FOR PROVIDING BROADCAST SERVICES ON SPOT BEAM SATELLITES,”filed on Mar. 21, 2007, the contents of which are hereby incorporated byreference for all purposes.

BACKGROUND OF THE INVENTION

Spot beams satellites have become common on the Ka band and MobileSatellite Services (MSS) bands (S band and L band). The Ka band, the Lband and the S band are portions of the microwave band of theelectromagnetic spectrum. The Ka band ranges from approximately 18 to 40GHz, the L-band ranges from approximately 1 to 2 GHz, and the S-bandranges from approximately 2 to 4 GHz.

Spot beam satellites produce signals that are concentrated on a limitedgeographical area. Spot beams are typically used where a contentprovider wishes to restrict content to an intended reception area. Onlyreceivers within the limited geographical area covered by the spot beamare able to receive the content. Spot beam satellites may be used totransmit a variety of content, such as audio and video content. Forexample, satellite television providers may use spot beam satellites toprovide localized content to subscribers in different cities.

Due to the confined nature of the signals produced by each spot beam,spot beam satellites facilitate frequency reuse. Multiple beams from oneor more satellites may use the same frequency to transmit different datawithout interfering with one another, so long as each beam is focused ona different geographical area. As a result, receiver design may also besimplified in spot beam systems, as the receiver does not require logicfor distinguishing data received from multiple transmitters on the samefrequency. For example, returning to the satellite television exampledescribed above, a satellite television provider can transmit content tolocalized content to different cities using the same frequency. Eachcity would be covered by spot beams providing localized content for thatcity and subscribers in each city would be able to receive the contenton the same frequency. Thus, the satellite television provider couldprovide receivers configured to receive on the same frequency regardlessof the geographical location of the subscriber.

In contrast to spot beam satellites, broadcast satellites produce asingle large beam that covers a wide geographical area. Broadcastsatellites are typically used to provide the same content to a largenumber of users distributed over vast geographical areas, such as thecontinental United States (CONUS). Broadcast satellites, like spot beamsatellites, may be used to transmit a variety of content, such asbroadcast audio and video.

A typical satellite has a functional lifetime of approximately 15 yearsand requires a large investment in resource in order to construct, placein orbit, and maintain the satellite. Accordingly, systems and methodthat enable the mission of a spot beam satellite to be changed from spotbeam transmission to broadcast transmission, and vice versa, withoutrequiring that the satellite be reconfigured are desired.

BRIEF SUMMARY OF THE INVENTION

Techniques for providing broadcast services on a spot beam satellite areprovided. Embodiments also enable the mission of a spot beam satellitesystem to be changed from providing spot beam transmission to broadcasttransmissions, and vice versa, without requiring that the satellite bereconfigured.

According to an embodiment, a satellite system for providing broadcastservice is provided. The satellite system includes a spot beam satelliteoperable in a first mode to provide broadcast data to a broadcastcoverage area via a plurality of spot beams, the broadcast coverage areacomprising a plurality of spot beam coverage areas. The satellite isalso operable in a second model to provide spot beam transmission to theplurality of spot beam coverage areas. According to some embodiments,when the satellite is operating in the first mode, the broadcast data isspace-time encoded and each spot beam transmits the broadcast data usingthe same frequency.

According to another embodiment, a broadcast signal source for providingbroadcast data to a spot beam satellite broadcasting system is provided.The broadcast signal source includes a transmitter configured totransmit a plurality of uplink signals to a spot beam satellite. Each ofthe uplink signals corresponds to a spot beam of the spot beamsatellite. The broadcast signal source also includes a data encodermodule for encoding the broadcast data such that the spot beam satellitemay simultaneously broadcast the encoded data on at least a portion ofthe plurality of spot beams using the same frequency. According to someembodiments, the data encoder encodes the broadcast data using aspace-time code.

According to yet another embodiment, a method for broadcasting datausing a spot beam satellite system is provided. The method includesencoding the broadcast data to provide a plurality of encoded datasignals and broadcasting the data using the plurality of spot-beams ofthe spot beam satellite. Each of the plurality of encoded data signalscorresponds to one of a plurality of spot beams of a spot-beamsatellite. The data is encoded such that the spot beam satellite maysimultaneously broadcast the encoded data on at least a portion of theplurality of spot beams using the same frequency on each of the spotbeams. According to an embodiment, encoding the broadcast data includesencoding the broadcast data using a space-time code to produce aplurality of encoded outputs, and each of the encoded outputscorresponds to a spot beam of the spot beam satellite.

Other features and advantages of the invention will be apparent in viewof the following detailed description and preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of a spot beam satellite system providingbroadcast coverage from a single satellite according to an embodiment ofthe present invention.

FIG. 2 is a block diagram of a conventional encoder for encodingbroadcast data for a spot beam broadcast satellite system.

FIG. 3 is a block diagram of a space-time encoder for encoding broadcastdata for a spot beam broadcast satellite system according to anembodiment of the present invention.

FIG. 4 is another illustration of a spot beam satellite system providingbroadcast coverage from spot beam satellite according to an embodimentof the present invention.

Embodiments of the invention are described here, with reference to thefigures. Where elements of the figures are called out with referencenumbers, it should be understood that like reference numbers refer tolike elements and might or might not be the same instance of theelement.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention advantageously provide systems andmethods for providing broadcast services on spot beam satellites.Embodiments advantageously provide a flexible solution with satellitesthat may be reconfigured for either wide-beam or spot-beam transmissionsusing spot beam satellites.

Embodiments advantageously combine space-time coded transmissions withspot beam satellite communications to distribute broadcast content overa wide geographical region using a spot beam satellite system. In aspecific embodiment, a satellite broadcast system comprises N spot beamsand a space-time encoder with N outputs. Each of the N space-timeencoder outputs is transmitted simultaneously on different spot beamsusing the same frequency. Spot beam coverage areas may be contiguousgeographical locations. Each user terminal may receive the broadcastsignal from a plurality of spot beams at widely varying receive signalpower levels, and the user terminal performs space-time decoding todecode the broadcast content without interference from adjacent ornearby beams.

FIG. 1 is an illustration of a spot beam satellite system 100 providingbroadcast coverage from a single satellite 110 according to anembodiment. Satellite 110 transmits a plurality of spot beams with eachof the spot beams providing coverage over a limited geographical area.Satellite 110 is configured to use the same frequency for a number ofbeams in the intended coverage area.

Spot beam coverage areas 105(1)-105(N) together comprise some or all ofthe broadcast coverage area for satellite 110. As illustrated in FIG. 1,spot beam coverage areas 105(1)-105(N) may advantageously comprisecontiguous geographical areas using the same or overlapping frequencybands, unlike conventional spot beam broadcasts where spot beamtransmissions to coverage areas comprising contiguous geographical areasmay result in signal interference from adjacent or nearby beams on thesame frequency.

Gateway 120 comprises a space-time encoder (not pictured) that isconfigured to use a space-time code (STC) to encode the broadcast data.The STC encoder is configured to have N outputs, one for each of the Nbeams. Each output is separately modulated on its own carrier using amodulation such as MPSK and QAM. One skilled in the art will recognizethat other modulation techniques may also be used.

According to an embodiment of the present invention, the N modulatorsare run synchronously so that the N output signals on each of the N spotbeams are symbol synchronized. The synchronization of the modulators iseasier if all of the modulators reside in the same physical location.However, according to some embodiments of the present invention, atleast a portion of the modulators may reside at different physicallocations and various techniques, such as using a synchronizationsignal, may be used to synchronize the modulators.

The output of each of the modulators is up-converted and transmitted tosatellite 110 via beam uplink signals 115(1)-115(N). Each beam uplinksignal corresponds to one of the N spot beams 105(1)-105(N). Satellite110 receives beam uplink signals 115(1)-115(N) and retransmits thesignals received as downlink signals via spot beams 105(1)-105(N). Thedownlink signals 105(1)-105(N) occupy the same frequency and thedownlink signals at least partially overlap in time. Satellite 110transmits broadcast data on the same frequency on each of the beams inthe intended coverage area, and because the broadcast signals arespace-time encoded, signals from adjacent and/or nearby spot beams willnot cause interference at receivers within the coverage area of aparticular spot beam.

According to some embodiments, satellite 110 is a bent pipe satellitethat retransmits broadcast data received from a broadcast signal source,such as gateway 120. However, according to other embodiments, satellite110 may perform additional processing on the broadcast data prior toretransmission, such as frequency translation and/or signalamplification.

FIG. 2 is a block diagram of a conventional encoder 200 for encodingbroadcast data for a spot beam broadcast satellite system. Conventionalencoder 200 includes FEC encoders 210(1) through 210(N), modulators230(1)-230(N), and uplink transmitter 240. Conventional encoder 200requires that spot beams having nearby or adjacent coverage areastransmit on different frequencies or at different time intervals if theusing the same frequencies. Otherwise, if spot beams with nearby oradjacent coverage areas transmit on the same frequencies at the sametime, the signals from nearby or adjacent beams are likely to causeinterference at receivers within the coverage areas of the adjacent ornearby beams.

Conventional encoder 200 receives broadcast input signals 205(1)-205(N)comprising the content to be broadcast by the plurality of spot beams ofa spot beam satellite. Forward Error Correction (FEC) encoder modules210(1)-210(N) each receive broadcast input signals 205(1) through 205(N)and perform FEC encoding on the input signal. FEC encoding addsadditional redundant data to the signal that enables a receiver todetect and correct some errors in the signal received from the satellitesystem.

FEC encoder modules 210(1)-210(N) outputs an FEC encoded signal and theencoded signal is input to one of modulator modules 230(1)-230(N).Modulator modules 230(1)-230(N) modulate the encoded signals received asinputs on separate carriers and output the modulated signals. Variousmodulation techniques such as M-ary Phase Key Shifting (MPSK) orquadrature amplitude modulation (QAM) may be used. One skilled in theart will recognize that other modulation techniques may also be used.

Uplink transmitter 240 receives each of the modulated signals output bymodulator modules 230(1) through 230(N). Uplink transmitter 240transmits a separate uplink signal 250(1) through 250(N) to a satellite,with each uplink signal corresponding to a different spot beam.

Conventional encoder 200 may implement a conventional four-color unicastapproach that is commonly used in spot beam satellite systems. Accordingto the four-color unicast approach, the satellite bandwidth ispartitioned into four disjoint frequency segments with each of thesegments equal to one quarter of the bandwidth of the total frequencyallocation. Sometimes separate antenna polarization is used to doublethe available frequency allocation. Thus, transmissions for each spotbeam are confined to only a subset of the bandwidth available to thesatellite.

FIG. 3 is a block diagram of a space-time encoder for encoding broadcastdata for a spot beam broadcast satellite system according to anembodiment. Space-time encoder 300 includes FEC encoder 310, ST encoder320, modulators 330(1)-330(N), and uplink transmitter 340. Space-timeencoder 300 advantageously enables signals from all of the spot beams totransmit on the same frequency at the same time without signals fromnearby or adjacent beams causing interference at receivers with aparticular spot beam's coverage area.

Space-time encoder 300 advantageously provides more bandwidth for signaltransmission than the conventional four-color unicast approach that iscommonly used in spot beam satellite systems (described above). Incontrast, embodiments of the present invention enable each spot beam totransmit using the full bandwidth available to the satellite.

Space-time encoder 300 also advantageously eliminates spot-beaminterference, which commonly occurs as a side effect in unicast systemsthat include frequency reuse. According to embodiments, the broadcastsignals transmitted on the plurality of spot beams of the satellitesystem are encoded to avoid interference from adjacent and/or nearbybeams.

Space time encoder 300 receives a broadcast input signal 305 comprisingthe content to be broadcast by the plurality of spot beams of a spotbeam satellite, such as spot beam satellite 110 described above. ForwardError Correction (FEC) encoder module 310 receives broadcast inputsignal 305 and performs FEC encoding on the input signal. FEC encodingadds additional redundant data to the signal that enables a receiver todetect and correct some errors in the signal received from the satellitesystem. Retransmission of data to individual receivers would beimpractical in a broadcast satellite communication system, such as themodified spot beam satellite communication system of the subjectinvention, because the same data is concurrently broadcast by aplurality of spot beams and the spot beam coverage area of each spotbeam may include a plurality of receivers. By FEC encoding the broadcastdata, the system provides receivers with the ability to correct at leastsome errors in the received signal.

FEC encoder module 310 outputs an FEC encoded signal and the encodedsignal is input to space-time (ST) encoder module 320. ST encoder module320 applies a space-time code to the FEC encoded data. ST encoder module320 may use various space-time codes such as the Alamouti code,described below.

ST encoder module 320 produces N outputs, each of the N outputscorresponding to one of the N spot beams of the spot beam satellite.Space time encoder 300 includes N modulator modules, modulator modules330(1)-330(N), with each of the modulator modules corresponding to oneof the N spot beams of the satellite. Modulator modules 330(1)-330(N)each receive one of the N outputs from ST encoder module 320. Modulatormodules 330(1)-330(N) modulate the encoded signals received as inputs onseparate carriers and output the modulated signals. Various modulationtechniques such as M-ary Phase Key Shifting (MPSK) or quadratureamplitude modulation (QAM) may be used. One skilled in the art willrecognize that other modulation techniques may also be used.

Uplink transmitter 340 receives each of the modulated signals output bymodulator modules 330(1) through 330(N). Uplink transmitter 340transmits a separate uplink signal 350(1) through 350(N) to satellite110. Uplink signals 330(1) through 330(N) each correspond to a separatespot beam coverage area of spot beam coverage areas 105(1) through105(N) illustrated in FIG. 1.

FIG. 4 is another illustration of a spot beam satellite system 400providing broadcast coverage from spot beam satellite according to anembodiment. Spot beam satellite system 400 comprises satellite 430 thatincludes two spot beams providing broadcast coverage to two spot beamcoverage areas 480(1) and 480(2). One skilled in the art will recognizethat other embodiments may include spot beam satellite systems thatinclude satellites that provide more than two spot beams.

ST encoder 410 uses the Alamouti code as the space-time coding methodfor encoding broadcast input signal 401. Broadcast input signal 401includes the content to be broadcast to receivers within spot beamcoverage areas 480(1) and 480(2). ST encoder 410 outputs two encodedsignals, one signal for each of the beams. The outputs from ST encoder410 are input into modulator and transmitter module 420. Modulator andtransmitter module 420 performs similar functions as modulators330(1)-330(N) and uplink transmitter 340 described above. Modulator andtransmitter module 420 modulates the space-time coded signals receivedfrom ST encoder module 410 and transmits an uplink signal correspondingto each beam to satellite 430. Satellite 430 receives the uplink signalsand retransmits the data in the uplink signals as downlink signals oneach of the two beams. Bandwidth utilization map 470 illustrates thatbeams 1 and 2 are both able to simultaneously utilize the full bandwidthof W Hz available to satellite 430.

ST encoder 410 employs the Alamouti code, which is designed for a twoantenna transmitter system. One skilled in the art will recognize thatother space-time coding method may be used in other embodiments. TheAlamouti code is a rate-1 code, meaning that it will take two timeslotsto transmit two symbols in a block S1 and S2. Symbols S1 and S2represent complex valued coordinates for a particular M-ary QAM, PSK, oramplitude phase-shift keying (APSK) constellation (of which, binaryphase-shift-keying (BPSK), Quadrature Phase Shift Keying (QPSK), and 8Phase Shift Keying (8-PSK) are special cases).

At the first symbol transmission interval, the signal transmitted tofirst beam 485(1) will be S₁ and the signal transmitted to second beam485(2) will be S₂. During the second signaling interval in the block of2, the signal transmitted to first beam 485(1) will be −S2* and thesymbol transmitted to second beam 485(1) will be SI* where * indicatescomplex conjugation.

User Terminal (UT) 450 is located within the footprint of second beam485(2) but near the edge of beam 485(1). However, since the downlinksignals 485(1) and 485(2) occupy the same bandwidth at the same time,the signal received by UT 450 will have contributions from both beamsignals. Specifically, the signal out of the matched filters in the UTsampled at the optimal time is:First Symbol: r ₁ =h ₁ S ₁ +h ₂ S ₂ +N ₁  (equation 1)Second Symbol: r ₂ =−h ₁ S ₂ *+h ₂ S ₁ *+N ₂  (equation 2)where h₁ and h₂ are assumed to be constant over a two symbol period andh₁ and h₂ represent the channel gain from each of the two beams to theUT, and N₁ and N₂ are additive white Gaussian Noise (AWGN). The channelgains consist of a magnitude and a phase, h₁=α₁e^(jθ) ¹ and h₂=α₂e^(jθ)² . The received power from each beam is proportional to α₁ and α₂,respectively. The magnitude of the channel gains α₁ and α₂ depend on thelocation of the UT within the coverage area comprising the footprint ofthe first and second beams.

For a UT near the center of first beam 485(1), α₁ will be large and α₂will be small. For a UT in near the center of second beam 485(2), α₂will be large and α₁ will be small. For UT's near the edge that bordersboth beams, the beam crossover points, both α₁ and α₂ will be about thesame value and smaller, sometimes significantly smaller, than the peakvalues that occur at the center of the beams. Beam rolloff may be to be3 dB or more at the beam crossover points. In spot beam unicast systems,UT locations at the beam crossover points are typically the worstlocations for spot beam unicast traffic. However, embodiments of thespot broadcast satellite system advantageously overcome this problem,enabling the system to broadcast content to contiguous spot beamcoverage areas using the same frequency without experiencing loss ofsignal quality for UTs in or near beam crossover points.

Within the UT, space-time decoding is performed by multiplying thereceived signals by the channel gains, which in the present embodiment,must be known to the decoder,Ŝ ₁ =h ₁ *r ₁ +h ₂ r ₂*=(α₁ ²+α₂ ²)S ₁ +h ₁ *N ₁ +h ₂ N ₂*  (equation 3)Ŝ ₂ =h ₂ *r ₁ −h ₁ r ₂*=(α₁ ²+α₂ ²)S ₂ +h ₂ *N ₁ −h ₁ N ₂*  (equation 4)As demonstrated in equations 3 and 4 above, the ideal decoder outputaccording to embodiments of the present invention has the twotransmitted symbols completely separated so there is no beaminterference. Furthermore, the received energy in each of the decodedoutputs is proportional to the sum of the received energy from firstbeam 485(1) and second beam 485(2). This property, referred to adiversity gain, advantageously enables UTs located at beam crossoverpoints to operate without experiencing energy loss due to beam rolloff.Also, the variation of the signal-to-noise ratio (SNR) also may beadvantageously reduced. Reducing variations in the SNR of a broadcastchannel may also increase the capacity of the broadcast channel,enabling better quality communication because the capacity of abroadcast channel may be defined by the SNR of UTs that fall withinworst case locations within the coverage area, such as the beamcrossover points.

Spot beam satellite system 400 described above includes an orthogonalspace-time code of rate=1 and N=2 outputs. One skilled in the art willrecognize that other embodiments may be implemented using otherorthogonal codes or other non-orthogonal codes.

The use of orthogonal codes in spot beam broadcast satellite systemsprovide a number of advantages. These benefits include, but are notlimited to (1) no spot beam interference, and (2) the decoded signalenergy is proportional to the sum of the channel gains (magnitudesquared) for all spot beams. Another advantage of incorporatingorthogonal codes into embodiments of the spot broadcast satellite systemis that the decoding process for orthogonal codes is relatively simpleand results in soft outputs. The soft outputs from the orthogonaldecoder may then be input into a space-time decoder. The soft outputsalso enable powerful forward error correction (FEC) techniques to beapplied to the signal (FEC is well known as discussed above).Embodiments may use various powerful FEC coding techniques, such as, butnot limited to, Turbo Codes, and Low Density Parity Check Codes.Furthermore, according to other embodiments, the N=2 output orthogonalspace-time code can be extended to any arbitrary number of outputs,however the code rate for these embodiments will be less than one butwill not be less than one half. Furthermore, the codeword block lengthwill be no larger than 2N. All of the desired properties described aboveare retained.

One skilled in the art will recognize that embodiments are not limitedonly to orthogonal space-time codes. Embodiments may advantageously useany space-time code so long as the number of encoder outputs correspondsto the number of beams in the coverage area. Some alternativeembodiments may include non-orthogonal codes, quasi-orthogonal codes,trellis space-time codes, super-orthogonal space-time trellis-codes,turbo space-time codes, super-orthogonal space-time codes, and/or otherspace-time codes that provide the same number of outputs as the numberof spot beams that comprise the coverage area of the spot beam broadcastsatellite system.

Many of the STCs described above require knowledge of the channel gain,which is also known as channel state information (CSI). Various methodsare known to the art for obtaining the CSI, such as transmitting a knownpilot symbols to perform channel sounding to measure the channel gains.The known pilot symbols may be realized in various ways, including, butnot limited to (a) as periodic pilot symbols multiplexed into the symbolstream on a time-division multiplexing (TDM) basis, (b) as an adjacentsmall bandwidth FDM carrier that transmits only a known symbol stream,or (c) as pilot signals on orthogonal PN code for use within a codedivision multiple access (CDMA) system. Embodiments may implement one ormore of the CSI methods described herein and/or others.

According to some embodiments, a differential space-time code isemployed rather than performing a channel gain estimate. A non-coherentdifferential STC allows decoding of the encoded signal to be performedwithout knowledge of channel gains. Using a differential STC eliminatesthe need to estimate the gain for each of the beams. However, decoding anon-coherent code may be more computationally intensive than performinga channel gain estimate as described above.

Construction and decoding techniques of the various STCs describedherein are known in the art. One reference that discusses the STCsdescribed herein is “Space-Time Coding Theory and Practice,” by HamidJafarkhani, published by Cambridge University Press (3005) (ISBN-10:0521842913; ISBN-13: 978-0521842914). However, the application of STC ina spot beam satellite system to enable a spot beam satellite tobroadcast data on the same frequency on a plurality of spot beams asdescribed herein is novel application of STCs that provides for flexiblesatellite systems that may be configured to provide either broadcastand/or spot beam transmissions from the same satellite system.

According to an embodiment of the present invention, satellite system100 may comprise a constellation of low earth orbit (LEO) satellites110, and gateway 120 is configured to use a use all of the spot beams(or at least a subset of the spot beams) of the constellation ofsatellites 110 to provide coverage over fixed broadcast area. Satellites110 are in a low earth orbit, which means that the beams move relativeto the surface of the Earth, in contrast with spot beam satellites in ageosynchronous orbit where the angular velocity of the satellites matchthe angular velocity of the Earth and the spot beams stay in a fixedposition relative to the surface of the Earth. Accordingly, theindividual spot beams and the number of spot beams that provide coverageto the fixed broadcast area will vary with time. As the number of beamsproviding coverage to the fixed broadcast area changes, space-timeencoder 300 is dynamically reconfigured, such that ST encoder module 320produces N₁ outputs, each of the N₁ outputs corresponding to one of theN₁ spot beams providing coverage to the fixed broadcast area at a firstpoint in time and ST encoder module 320 produces N₂ outputs, each of theN₂ outputs corresponding to one of the N₂ spot beams providing coverageto the fixed broadcast area at a second point in time.

According to another embodiment of the present invention, satellitesystem 110 may comprise Ka-band spot beam satellites 110. Manyconventional Ka-band spot beam satellites have beams configured in4-color or 7-color frequency reuse pattern. Adjacent beams are typically“hardwired” to operate on different frequencies to prevent adjacentbeams from transmitting on the same frequency. However, according to anembodiment of the present invention, the techniques for providingbroadcast services on spot beam satellites described above may beutilized on a Ka-band satellite system. The space-time encodingtechnique described above may be used for all beams that operate on thesame frequency, and the broadcast is repeated for all frequencies atwhich beams of the Ka-band satellite system are operating. Thistechnique helps to eliminates spot beam interference, but does not allof the beams to use the full bandwidth available to the satellite as inthe embodiments described in FIGS. 1, 3, and 4. Furthermore, in atypical Ka-band satellite system, spot beams operating on the samefrequency are generally connected to hubs in different physicallocations. As a result, space-time encoding is performed at each of thehubs, but the hub uses only the STC output corresponding the hub's spotbeam operating at a particular frequency. The transmissions from each ofthe hubs are pre-corrected so that the transmissions arrive at thesatellite symbol synchronized.

While the embodiments described above may make reference to specifichardware components, those skilled in the art will appreciate thatdifferent combinations of hardware and/or software components may alsobe used and that particular operations described as being implemented inhardware might also be implemented in software or vice versa.

Computer programs incorporating various features of the presentinvention may be encoded on various computer readable media for storageand/or transmission; suitable media include magnetic disk or tape,optical storage media such as compact disk (CD) or DVD (digitalversatile disk), flash memory, and the like. Such programs may also beencoded and transmitted using carrier signals adapted for transmissionvia wired, optical, and/or wireless networks conforming to a variety ofprotocols, including the Internet. Computer readable media encoded withthe program code may be packaged with a compatible device or providedseparately from other devices (e.g., via Internet download).

Thus, although the invention has been described with respect to specificembodiments, it will be appreciated that the invention is intended tocover all modifications and equivalents within the scope of thefollowing claims.

What is claimed is:
 1. A satellite system comprising: a spot beamsatellite operable in a first mode to provide broadcast data to abroadcast coverage area via a plurality of spot beams and operable in asecond mode to provide spot beam transmissions via the plurality of spotbeams to a plurality of spot beam coverage areas, wherein the spot beamsatellite transmits using a first frequency to a first spot beamcoverage area via a first spot beam and to a second spot beam coveragearea via a second spot beam of the plurality of spot beams, and whereinthe first spot beam coverage area and the second spot beam coverage areacomprise contiguous geographical areas; a first hub configured totransmit first signals associated with the first spot beam to the spotbeam satellite; and a second hub configured to transmit second signalsassociated with the second spot beam to the spot beam satellite, whereinthe second hub is located in a different physical location than thefirst hub, wherein, when the spot beam satellite is operating in thefirst mode, the first and second signals comprise space-time codedsignals of a same input broadcast data, and wherein timing of the firstand second signals are pre-corrected to be symbol-synchronized whenreceived at the spot beam satellite, and wherein when the spot beamsatellite is operating in the second mode, the first and second signalscomprise different data.
 2. The system of claim 1, wherein when the spotbeam satellite is operating in the first mode, each of the plurality ofspot beams transmits the broadcast data on the same frequency at thesame time to the plurality of spot beam coverage areas comprising thebroadcast coverage area.
 3. The system of claim 1, wherein the inputbroadcast data is space-time encoded using an orthogonal space-timecode.
 4. The system of claim 1, wherein the input broadcast data ismodulated such that the data transmitted on the plurality of spot beamsis symbol synchronized.
 5. The system of claim 4, wherein the broadcastdata is forward error correction (FEC) encoded to enable receiverswithin the broadcast coverage area to correct errors in the broadcastdata received by the receivers.
 6. The system of claim 5, wherein thespot beam satellite receives the broadcast data from a broadcast signalsource via a plurality of uplink signals, wherein the plurality ofuplink signals comprises a separate uplink signal for each of theplurality of spot beams.
 7. The system of claim 6, wherein the broadcastdata is modulated, space-time encoded, and FEC encoded at the broadcastsignal source.
 8. The system of claim 1, wherein the spot beam satelliteis one of a constellation of low earth orbit (LEO) satellites comprisingthe satellite system, wherein one or more spot beams from one or more ofthe constellation of LEO satellites provide the broadcast data to thebroadcast coverage area, and wherein the one or more spot beams from theone or more of the constellation of LEO satellites that provide thebroadcast data vary with time.
 9. The system of claim 8, whereinspace-time encoding used to encode the input broadcast data isdynamically updated to produce encoded output signals corresponding toone or more spot beams from one or more of the constellation of LEOsatellites currently providing the broadcast data to the broadcastcoverage area.
 10. The system of claim 1, wherein, when the spot beamsatellite is operating in the first mode, the spot beam satellitetransmits at the first frequency using a first subset of the pluralityof spot beams and transmits at a second frequency using a second subsetof the plurality of spot beams, and wherein the input broadcast data isseparately encoded using a space-time code for the first subset of theplurality of spot beams and for the second subset of the plurality ofspot beams.
 11. The system of claim 1, wherein, when the spot beamsatellite is operating in the first mode, the spot beam satellitetransmits over a full bandwidth available to the spot beam satellite foreach of the plurality of spot beams.
 12. A method for providingbroadcast services in a satellite system, comprising: providing spotbeam transmissions to a plurality of spot beam coverage areas via aplurality of spot beams of a spot beam satellite, wherein the spot beamsatellite is operable in a first mode to provide broadcast data to abroadcast coverage area via the plurality of spot beams and operable ina second mode to provide spot beam transmissions via the plurality ofspot beams, wherein the spot beam satellite transmits using a firstfrequency to a first spot beam coverage area via a first spot beam andto a second spot beam coverage area via a second spot beam of theplurality of spot beams, and wherein the first spot beam coverage areaand the second spot beam coverage area comprise contiguous geographicalareas; transmitting first signals associated with the first spot beam tothe spot beam satellite from a first hub and second signals associatedwith the second spot beam to the spot beam satellite from a second hub,wherein the second hub is located in a different physical location fromthe first hub, wherein, when the spot beam satellite is operating in thefirst mode, the first and second signals comprise space-time codedsignals of a same input broadcast data, and timing of the first andsecond broadcast data signals are pre-corrected to besymbol-synchronized when received at the spot beam satellite, andwherein, when the spot beam satellite is operating in the second mode,the first and second signals comprise different data.
 13. The method ofclaim 12, wherein when the spot beam satellite is operating in the firstmode, each of the plurality of spot beams transmits the broadcast dataon the same frequency at the same time to the plurality of spot beamcoverage areas comprising the broadcast coverage area.
 14. The method ofclaim 12, wherein the input broadcast data is space-time encoded usingan orthogonal space-time code.
 15. The method of claim 12, wherein theinput broadcast data is modulated such that the data transmitted on theplurality of spot beams is symbol synchronized.
 16. The method of claim12, wherein the spot beam satellite is one of a constellation of lowearth orbit (LEO) satellites comprising the satellite system, whereinone or more spot beams from one or more of the constellation of LEOsatellites provide the broadcast data to the broadcast coverage area,and wherein the one or more spot beams from the one or more of theconstellation of LEO satellites that provide the broadcast data varywith time.
 17. The method of claim 16, wherein space-time encoding usedto encode the input broadcast data is dynamically updated to produceencoded output signals corresponding to one or more spot beams from oneor more of the constellation of LEO satellites currently providing thebroadcast data to the broadcast coverage area.
 18. The method of claim12, wherein, when the spot beam satellite is operating in the firstmode, the spot beam satellite transmits at the first frequency using afirst subset of the plurality of spot beams and transmits at a secondfrequency using a second subset of the plurality of spot beams, andwherein the input broadcast data is separately encoded using aspace-time code for the first subset of the plurality of spot beams andfor the second subset of the plurality of spot beams.
 19. The method ofclaim 12, wherein, when the spot beam satellite is operating in thefirst mode, the spot beam satellite transmits over a full bandwidthavailable to the spot beam satellite for each of the plurality of spotbeams.